Photodetectors are comprised of a diode that permits an amount of current to flow through it that is proportional to the intensity of light with which it is illuminated and a readout circuit for producing a voltage corresponding to the current flowing through the diode and hence also corresponding to the intensity of the light. Diodes using InGaAs are superior to those using Gi, HgCdTe,PtSi or InSb for certain applications because of their higher sensitivity, faster operation and the fact that they function well at room temperature.
If an InGaAs diode were epitaxially grown on the Si substrate on which the readout circuitry is preferably formed, the lattice mismatch would have so many dislocations as to cause sufficient leakage current to seriously degrade diode performance. For this reason a hybrid structure has been used for several years wherein the diode is separate from the substrate and coupled to its readout circuit thereon by wire or Indium bump bonds. Unfortunately, however, such connections are susceptible to vibration failure in a space launch and are plagued by parasitic capacitance and inductance that lower their reliability and decrease their bandwidth. Furthermore, extensive packaging efforts are required in order to form an array of such detectors.
As reported in Journal Applied Physics (Vol. 65(6), pg. 2220-2237, 1989), E. A. Fitzgerald et al have selectively grown In.sub.0.05 Ga.sub.0.95 As on GaAs substrates with very few dislocations by confining the epitaxial growth to a location that is square or circular having an area of (100.times.100).mu.m.sup.2 or less.
It is also known that if a desired epitaxial layer has a lattice that is severely mismatched with the lattice of a substrate on which it is epitaxially grown as to severely limit the operation of a device of which it is a part. In such a case, the desired epitaxial layer can be compositionally graded to the substrate via successive epitaxial layers in which the ratio of the elements respectively changes from that of the desired composition to a composition having far less lattice mismatch with the substrate.